With regard to this kind of device, a conventional technique is known which includes obtaining a first control threshold based on a first degree of risk at the left side of a vehicle and a second control threshold based on a second degree of risk at the right side of the vehicle (See JP5070171B). The first control threshold and the second control threshold each represent a distance from a road end. The first control threshold is changed toward a first object at the left side of the vehicle as the second degree of risk increases while the second control threshold is changed toward a second object at the right side of the vehicle as the first degree of risk increases.
In the above conventional technique, however, when no object exists at the right and left of a subject vehicle, the subject vehicle is controlled to travel constantly at the center of a travel lane. Therefore, when the subject vehicle changes its course at a branch, such as an intersection, the subject vehicle is to travel at the center of a travel lane until just before changing the course and suddenly changes the course at the branch. Such a sudden course change diverges from the passengers' expectation of following the course change motion and gives an uncomfortable feeling to the passengers, which may be problematic.